The most common and most likely answer to this crossword clue is the 3 plus 5 letter word LOG CANOE
I've seen this crossword clue in Financial Times
L
O
G
C
A
N
O
E
Buddy explains!
The straight part
We know that "primitive mode of transport" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case primitive mode of transport = log canoe.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.